Refrigeration systems including a refrigerant circuit operating in a refrigeration cycle by circulating refrigerant therethrough have conventionally been widely used, such as for air conditioners.
For example, Patent Document 1 discloses an air conditioner including a twin-cylinder compressor. The refrigerant circuit of this air conditioner is provided with a compressor, an indoor heat exchanger, an expansion valve, an outdoor heat exchanger and other components. The compressor includes a drive motor, a drive shaft that can be driven by the drive motor, and first and second compression mechanisms connected to the drive shaft. The two compression mechanisms are composed of so-called rotary compression mechanisms in which a piston eccentrically rotates in the cylinder chamber in a cylinder. In other words, each compression mechanism constitutes a positive-displacement fluid machine in which the capacity of a compression chamber for refrigerant formed in the cylinder chamber cyclically changes.
In this air conditioner, the compression mode of the compressor can be changed by changing the flow path of refrigerant depending on the operating conditions. Specifically, the compressor of this air conditioner can be switched among a parallel compression mode, a cylinder nonoperating mode and a two-stage compression mode.
In the parallel compression mode, refrigerant flow is distributed to the first and second compression mechanisms and refrigerant is compressed in a single stage in each of the compression mechanisms. In the cylinder nonoperating mode, refrigerant is compressed only in the first compression mechanism and is not compressed in the second compression mechanism. In the two-stage compression mode, refrigerant is first compressed in the first compression mechanism and then further compressed in the second compression mechanism. In other words, in the two-stage compression mode, refrigerant is compressed in two stages in such a manner that the first compression mechanism is used as a low-pressure stage compression mechanism and the second compression mechanism is used as a high-pressure stage compression mechanism.    Patent Document 1: Published Japanese Patent Application No. S64-10066